Antioxidants of 15 onions with white, yellow, and red colors and their relationship with pungency, anthocyanin, and quercetin

We have investigated the antioxidant activity (AOA) of 15 short-day onions with white, yellow, or red colors and elucidated the relationships between pungency, anthocyanins, quercetin, and the AOA levels. There were substantial variations in both the pungency and total soluble solid content, which showed varying responses by bulb colors. The AOA in white or red onions tended to have low and high levels, respectively. However, there were many exceptions. The AOA levels assessed by the Folin–Ciocalteu (F–C) assay ranged between 440 and 785 μg/mL. The AOA levels based on the 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay were between 19.1 and 79.8 μg/mL and displayed 10- to 20-fold differences. There was no correlation between the pungency and the AOA levels. The F–C assay showed no or low correlation with anthocyanin (r2 = 0.07) and quercetin (r2 = 0.59) contents, respectively. The DPPH assay showed correlations with anthocyanin (r2 = 0.65) and quercetin (r2 = 0.76) contents. Therefore, the onions with higher levels of anthocyanin and quercetin would have still higher AOA levels. Samples with alliinase action showed higher AOA levels than the samples without enzyme action. Onion juice without anthocyanin and flavonoids still contained considerable AOA levels. The methanol extract and fresh onion juices showed similar AOA levels by the F–C assay.